The present invention relates to communication systems and more particularly, to automatic gain control circuits used in radio communications systems.
In many radio receivers, an automatic gain control (AGC) circuit is used to control the dynamic range of a received signal. A typical AGC circuit may track changes in the received signal and set the gain of the receiver accordingly. Such an AGC circuit may be incorporated into a receiver used in, for example, a mobile station such as a cellular phone or a car-mounted mobile phone in a radio communications system such as a Code Division Multiple Access (CDMA) System.
The dynamic range in different parts of a radio receiver in a mobile station such as a hand-held terminal (xe2x80x9ccellular phonexe2x80x9d) or similar portable terminal may be of importance due to requirements related to power consumption. Portable terminals are typically battery powered and thus there is a ubiquitous concern for conserving power therein. It is well known in the art that by limiting the dynamic range of a received signal within the receiver, power conservation may be realized. For example, the dynamic range of a radio signal input to an A/D converter should be as limited as possible to enable the use of low resolution, low power A/D converters in the receiver. Furthermore, power consumption in analog amplifiers and filter sections is directly proportional to the dynamic range of the input signal. Hence, there is a universal need for efficient AGC circuits to limit the dynamic range of an input signal both in the analog and digital circuit domains.
A digital AGC circuit typically tracks changes in the received signal in order to optimize the use of available dynamic range. Signal power is generally favored as a control parameter since it most directly impacts power use within the receiver. In a typical receiver, changes in received signal power may be due to fast fading, shadowing (e.g., blocking of a signal by a building, and similar external interference anomalies), or the received signal power may change due to internal system power control events and due to the impact of internal interference sources. Moreover changes in received signal power may be due to frequency channel switching associated with various configurations of cellular topology.
Cellular radio communications systems often use hierarchical cell configurations to achieve advantages by allocating cell layout and frequency reuse based on user type. Such a Hierarchical Cell Structure is further described in U.S. Patent Application entitled xe2x80x9cTailored HCSxe2x80x9d, Ser. No. 08/872,065, filed Jun. 10, 1997 and incorporated herein by reference now U.S. Pat. No. 6,094,581. Since mobile users fall into different use categories such as fast moving and slow moving (e.g., automobile based mobile users and walking mobile users), different cell configurations may use different operational parameters such as frequency allocation within the different categories of cells which best accommodate slow moving and fast moving users. Generally, in a hierarchical cellular system cells for slow moving users may be a subset of larger cells designated for fast moving users and may be denoted as, for example, a xe2x80x9cpico cellxe2x80x9d, as is further described in U.S. Patent application entitled xe2x80x9cSelf Tuning Signal Strength Thresholdxe2x80x9d, Ser. No. 09/179,958, filed Oct. 28, 1998 incorporated herein by reference. However, since it is plausible that a fast moving user will become a slow moving user and vice versa, and to facilitate cell to cell handoffs during the normal course of operation, it is often necessary for mobile stations to monitor the same and/or other frequency channels in anticipation of cell changeovers or handoffs. And some systems exist in the prior art to adjust the AGC gain values when performing such monitoring in anticipation of handoff. One such system Is disclosed in U.S. Pat. No. 5,524,009 to Tuutijarvi et al. on Jun. 4, 1996. Tuutijarvi et al. disclose fast AGC setting using a received signal strength (RSS) measurement procedure. In Tuutijarvi et al. when a handoff command is sent by a base station to a mobile station, xe2x80x9cfreexe2x80x9d slots are used to measure signal strength to set an AGC in advance of switching channels in the mobile station. While such a system may improve handoff time, it does not actually change the time constant for the AGC circuit and may become overloaded in a receiver which is constantly monitoring other radio channels.
In a CDMA system, a received signal may consist of a desired signal and interference. Typically, interference dominates in a received signal, particularly when considering the use of wide spreading methods. Thus, an AGC circuit will track not only signal power but the power of the interference as well. In a CDMA system, a mobile station may be expected to perform measurements on multiple frequency channels in order to prepare for inter-frequency hand-over. At different frequency channels, received signal power may differ significantly due to different interference levels, different load, and possibly, different radio network topologies. A mobile station may further be expected to perform measurements on frequency channels associated with other radio communication systems in order to perform inter-system hand-overs. Regardless of the motivation for measurements, they should be performed as fast as possible so as, for example, not to disturb voice quality associated with an ongoing call, or not to risk losing the call connection entirely.
Situations may further arise in which a CDMA system deliberately changes the interference level and/or the desired signal level. For example, in some radio communication systems, a mobile station may switch to receive other signals for positioning purposes. Positioning may be accomplished in a mobile station by measuring signals at different frequencies from multiple base stations at a receiver within the mobile station. The measured signals together with known positions associated with the base stations from which the signals were received, are used to calculate the position of the receiver using known methods such as triangulation. During such measurements, it may be advantageous to reduce the power of interfering signals originating from the same base station as the desired signal or from adjacent base stations. Measurements may be performed repetitively and the mobile station may require several measurements to calculate and/or update its position, particularly when moving, thus leading to rapid switching between normal reception and position oriented measurement modes, hand-off modes, and the like. Identifying unsynchronized sources may further require a receiver to switch between different received signals as further described in related U.S. application Ser. No. 09/093,315, supra, incorporated herein by reference.
Problems arise however in a receiver when starting up an AGC circuit on a new frequency particularly when changing frequency channels abruptly. Because a typical AGC circuit contains a control loop and relies on filtering successive signal samples, it may takes a certain time period for the AGC circuit to settle. This may be referred to as the AGC xe2x80x9csettling timexe2x80x9d. It is desirable to minimize the AGC settling time so as to minimize the time during which the AGC-controlled signal level is not in the target range. If multiple changeovers, or frequency switches are occurring often or continuously, the AGC settling time may occupy a large percentage of the total switching time and become a significant impediment to maintaining signal quality and conserving power.
Therefore, it would be appreciated in the art to have an AGC circuit which measured and adjusted receiver gain as quickly as possible with minimum settling time. Such a circuit would allow power to be reduced while improving quality measures on an active call connection.
It is therefore an object of the present invention to provide a receiver having an improved digital AGC circuit which minimizes the time during which an AGC-controlled signal level is out of the target range.
It is a further object of the present invention to provide reduced settling time for a digital AGC circuit when switching between multiple received signals such that the dynamic range of received signals is limited by the digital AGC circuit as quickly as possible to achieve power savings.
Therefore, in accordance with one aspect of the present invention, the foregoing and other objects are achieved in a receiver having a digital AGC circuit for use in a cellular communication system in which the digital AGC circuit switches between multiple received signals. The receiver may include a frequency synthesizer for changing frequency coupled to a control processor. The control processor is coupled to a memory for storing AGC parameters. The signal input may receive multiple received signals on different frequency channels. For example, in a Hierarchical Cellular System, the multiple received signals can represent signals from various cells which are monitored, for example, for handover purposes. A controller coupled to the frequency synthesizer, memory and digital AGC circuit may effect switching from reception of a first one of the multiple received signals to a second of the multiple received signals. The controller may also effect switching between a first set of memorized AGC parameters and a second set of memorized AGC parameters, and between a first system state and a second system state. The controller may further apply a gain control parameter associated with the second system state to the digital AGC circuit which gain control parameter has been previously stored, for example, when the receiver was last switched to the second system state.
The digital AGC circuit, in accordance with an embodiment of the present invention may further include a digital filter characterized by a set of coefficients and internal states and the stored AGC parameters can include the set of coefficients and last known values of the internal states of the digital filter for each of a number of frequency channels and/or system states. A first set of parameters including filter coefficients may further be associated with a first system state and a gain value associated with a first one of the multiple received signals. A second set of parameters including a second set of filter coefficients may be associated with a second system state and a second gain value associated with a second one of the multiple received signals. The first gain value may be adjusted during reception of the first signal and the second gain value may be adjusted during reception of the second signal.
In an embodiment of the present invention, the controller may switch the receiver from receiving the second signal associated with the second system state, for example, back to receiving the first of the multiple received signals associated with the first system state, retrieve the previously memorized gain value associated with the first system state from the memory and apply the retrieved first gain value associated with the first system state to the digital AGC circuit, and may update and then memorize the second gain value associated with the second system state and the second received signal.
In an alternative embodiment of the present invention, the controller may further employ a linear predictor and the memory may be used to store one or more previous AGC parameters associated with each system state for use in linear prediction. Upon switching back to the first received signal associated with the first system state, the controller may provide the stored previous values associated with the first system state to the linear predictor to produce a present value for the first gain value which may then be applied to the digital AGC circuit such that the settling time for the digital AGC circuit is reduced with regard to the first one of the multiple received signals. The first system state may further include a timer value which allows the controller to determine. a length of time that the first AGC parameters have been stored in memory, retrieve the first gain value from the memory and apply the first gain value to the digital AGC circuit only if the length of time is below a predetermined threshold. Accordingly state values which have aged beyond their usefulness may not be applied. However, a default gain value may be applied to the digital AGC circuit if the length of time the gain value has been stored is above a predetermined threshold.
In yet another embodiment of the present invention, a controller may be configured to adapt the digital AGC circuit to a first received signal during a first measurement interval. After adapting to the first received signal, the digital AGC circuit may save a first set of parameters associated with the digital AGC circuit, the first input signal and the first measurement interval in a memory. The first set or parameters may include a system state as may be defined, for example, by reference or association, particulary in the memory with the first received signal, including for example a signal frequency or channel number, the first measurement interval including, for example, a timer value, and associating the system state with a state associated with the digital AGC circuit, including, for example, coefficients and parameters associated therewith. The controller, during the course of operation in accordance with the present invention, may be further configured to switch from a second input signal back to the first input signal at which time the controller may set the digital AGC circuit to a state derived from the first saved state. It will be appreciated that setting the digital AGC circuit with a state derived from the first saved state will have a beneficial impact on the settling time of the digital AGC circuit particularly when the characteristics of the first input signal have not changed significantly. The controller may further be configured to adapt the digital AGC circuit to the first input signal during a second measurement interval. It may be further appreciated that, for example, after adapting during a second and subsequent measurement intervals, the states associated with the digital AGC circuit, the first signal, and the second and subsequent measurement intervals may also be stored in a manner similar to that described above with reference to the first measurement interval.